Field of the Invention
The invention relates to a process for producing a ceramic layer from at least two precursors on a substrate, in particular a ceramic layer having ferroelectric, paraelectric or superconducting properties.
In semiconductor technology, increasing interest is being shown in the use of ceramic thin films. That group of substances includes compounds having superconducting, ferroelectric or dielectric properties with a high dielectric constant. The latter two groups of compounds are, in particular, advantageous for use as a storage dielectric in capacitors of an integrated circuit.
One example of a semiconductor circuit having a capacitor is a DRAM storage cell. In order to increase the integration density, the cell may be produced in the form of a so-called stacked capacitor cell, in which the storage capacitor is disposed above the associated selection transistor. Among other things, the choice of the capacitor dielectric has an essential effect on the space required for a capacitor of that type.
Conventional capacitors mostly use layers of silicon oxide or nitride, which have a dielectric constant of at most 8, as the storage dielectric. The paraelectric materials in that group of substances, for example BST (barium strontium titanate, (Ba,Sr)TiO.sub.3) and the like have a dielectric constant .epsilon.&gt;150 and therefore allow a smaller capacitor to be used for an equal capacitance.
Storage elements of that type, having a paraelectric material as the capacitor dielectric (DRAMs) lose their charge, and therefore their stored information, when the supply voltage is interrupted. Furthermore, because of the residual leakage current, conventional storage elements need to be continually refreshed (refresh time). Due to the different polarization directions, the use of a ferroelectric material as a storage dielectric permits the construction of a non-volatile memory, which does not lose its information when the supply voltage is interrupted and does not need to be refreshed constantly. The residual leakage current of the cell does not affect the stored signal. Examples of a ferroelectric material from that group of substances include PZT (lead zirconium titanate, Pb(Zr Ti)O.sub.3) and SBT (strontium bismuth tantalate, SrBi.sub.2 Ta.sub.2 C.sub.9) Since the production of those new ferroelectrics and paraelectrics generally takes place at high temperatures in an oxidizing atmosphere, a material compatible with those conditions is needed, in particular, for the first capacitor electrode. Pt, Ru, RuO.sub.2 or a similar material is conventionally used.
There are three essential methods known for the production of ceramic thin films: a sputtering process, a CVD process and a so-called sol-gel process. In the sol-gel process, metallorganic starting chemicals are generally dissolved in a nonpolar aromatic solvent (for example in xylene), then the solution is applied to the wafer and spun (spin-on process). The thin film of metallorganic molecules which is obtained in that way is subsequently converted into an oxide film in the presence of oxygen. That oxide film is transformed into the phase with the desired electrical properties during a subsequent heat treatment, which in the case of SBT is typically carried out in a temperature range of from 700 to 800.degree. C. In the case of an SBT layer, a lamellar perovskite phase with ferroelectric properties is formed, while BST or PZT involve a simple perovskite (heat treatment at 450-650.degree. C.). An example of a sol-gel process of that type is described in International Publication No. WO 93/12538. In that production process, the use of the usual solvents, in particular the nonpolar aromatic solvents, causes problems because of the toxicity and potential carcinogenic nature of the vapors.
A further production process is described in an article by Preston and Hartling in Integrated Ferroelectrics, 1992, Vol. 1, pages 89 to 98. In order to produce PLZT layers, the acetates of the corresponding metals are used as precursors, and water is used as solvent. One disadvantage of that process is the high polarity of water, which can lead to difficulties in wetting the substrate and therefore to variations in the layer thickness. There is also the risk of the ceramic layer flaking off.
A production process for SBT is also described in an article entitled "Formation of SrBi.sub.2 Ta.sub.2 O.sub.9 : Part I. Synthesis and Characterization of a Novel "Sol-Gel" Solution for Production of Ferroelectric SrBi.sub.2 Ta.sub.2 O.sub.9 Thin Films", by T. Boyle et al., in Journal of Material Research, Vol. 11, No. 9, Sept. 1996, pages 2274 to 2281. In that case, the precursor containing Ta and the precursor containing Sr are dissolved in acetic acid, while the precursor containing Bi is dissolved in pyridine. A disadvantage with that process is the use of two different starting solutions, which are mixed immediately before the wafer is coated. There is also the problem of aging of the starting solution containing the acetic acid. In that solution, the precursor containing Ta reacts with the acetic acid to form ethyl acetate and water. The water hydrolyzes the precursor containing Ta, with the result that tantalum oxide clusters with high molecular weight are formed. Over the course of time, a colloidal and later suspended Ta.sub.2 O.sub.5 is produced, which can be detected by a change in viscosity after about 1 week and turbidity after about 2 weeks. It is consequently not possible to store the precursor containing Ta in acetic acid for long periods of time.